Conventionally, backrests (seatbacks) of front seats of vehicles are tilted at an adjustable angle relative to seat cushions, thereby supplying comfortableness to occupants seated on a passenger's and a driver's seat. Further, as for vehicles of a 3-door version, backrests (seatbacks) of front seats are tilted at an adjustable angle relative to seat cushions in a way that the front seats are folded forward and occupants easily get on and off a rear space of the vehicle. More specifically, according to a seat reclining apparatus disclosed in Japanese Patent No. 2645583 (see FIGS. 3-6), a base plate is attached to a seat cushion frame, while a turn plate is attached to a seatback frame. A ring-shaped internal gear is provided at an inner side of the turn plate. A lock gear is provided at an inner side of the base plate and includes teeth at an outer periphery of the lock gear, the teeth which are engageable with the internal gear of the turn plate. More particularly, the lock gear is biased by a torsion spring in a radial direction so that the teeth of the lock gear are engaged with the internal gear of the turn plate. When the engagement between the internal gear of the turn plate and the lock gear is released, a tilting angle of the seat cushion (seat bottom) relative to the seatback is adjusted.
In the above-described seat reclining apparatus, there have been cases in which an operator manually operates an operation lever while being seated and leaning on the backrest (seatback) of the seat. A lock release operation is then implemented so as to move the lock gear towards the center of the base plate via a cam in response to the manual operation of the operation lever by an operator. While the operator is being seated and is leaning on the backrest of the seat, the lock gear has been engaged with the internal gear of the turn plate, the internal gear which has been forced rearward by the operator leaning on the seatback. However, once the lock release operation is implemented while the lock gear is in engagement with the internal gear of the turn plate, the lock gear is suddenly disengaged from the internal gear of the turn plate. Further, an abnormal noise occurs at the time of the disengagement therebetween. The followings are one of the possible reasons of the occurrences of this abnormal noise. As illustrated in FIGS. 9-12, a lock gear 103 is formed with external teeth 109 at an outer periphery thereof. A gear plate 101 is formed with internal teeth 107 at an inner periphery thereof. In proportion to the load applied to the seatback, the gear plate 101 is applied with load so as to rotate in a rear direction. The lock gear 103, which is in engagement with the gear plate 101, is therefore pushed so as to rotate in the rear direction. The lock gear 103 is hence inclined or tilted rearward from a normal position by a clearance t between each guide wall 105 and the lock gear 103. As a result, the internal tooth 107 is not sufficiently engaged with the corresponding external tooth 109, which is positioned at a front-end portion of the whole sequence of the external teeth 109. Meanwhile, the other internal tooth 107 is firmly engaged with the corresponding external tooth 109, which is positioned at a rear-end portion of the whole sequence of the external teeth 109. Here, the front-end portion is a front side from the rearward tilting direction of the seatback, i.e., is a front side to which the seatback is tilted rearward, while the rear-end portion is a rear side from the rearward tilting direction of the seatback, i.e., is a rear side to which the seatback is tilted forward.
Therefore, when the internal teeth 107 are fully disengaged from the external teeth 109, a pushing force from the internal teeth 107 of the gear plate 101 is transmitted to and concentrated on a tooth top of the single external tooth 109, which is positioned at the rear-end portion of the whole sequence of the external teeth 109. Accordingly, a type of frictional sound may occur when the internal teeth 107 are released or disengaged with a large impact under such concentrated circumstances.
JP2002-101993A discloses a seat reclining mechanism, which prevents generation of an abnormal noise even when an occupant leans on a backrest (backseat). A front inclined face of a front locking tooth appearing in the forefront location of a lock plate (lock gear) is formed by adding a predetermined extracting angle to a pressure angle. The forefront location of the lock plate corresponds to a rear-end portion in a rotation direction of the backseat. Therefore, an impact, which may occur at an event that internal teeth of a second bracket are released from engagement teeth, is reduced, which prevents abnormal noise occurrences.
As disclosed in JP2002-101993A, a pressure angle is increased at a tooth top of the lock plate (lock gear) by being added with an extracting angle, which leads to reduction in a tooth thickness, and a tooth height of the engagement tooth of the lock plate (lock gear). This may cause lowering of engagement rigidity between the internal teeth of the bracket and the engagement teeth.
The present invention has been made in view of the above circumstances, and provides a seat reclining apparatus, in which occurrences of abnormal noises are prevented even if a lock releasing operation is performed in a state where an occupant is leaning on a backseat, and in which engagement rigidity between a lock gear and a gear plate is not lowered.